Uninterruptible Power Supplies (UPS) are widely used to provide power to electronic components in the event that the alternating current (AC) utility input voltage fails. UPSs are now widely used, for example with computers, including but not limited to personal computers, workstations, mini computers, and mainframe computers, to insure that valuable data is not lost and that the computer can continue to operate notwithstanding temporary loss of the AC utility input voltage.
Referring to FIG. 1A, a simplified block diagram of an AC load powered by an AC utility input voltage is shown. As shown in FIG. 1, AC utility input voltage 100 is supplied to first plug 102 such as a wall socket into which an AC load 120 is plugged. In this conventional arrangement, if the AC utility input voltage fails, operation of the AC load 120 may stop.
Referring now to FIG. 1B, an uninterruptible power supply 110 is inserted between the first plug 102 of the AC utility input voltage 100 and the AC load 120. It will be understood that the UPS may be external to the AC load 120 so that the UPS 110 and AC load 120 are connected by a second plug 112 as shown in FIG. 1B. Alternatively, the UPS 110 may be integrally included as part of the AC load 120.
FIG. 1B also illustrates details of an AC load 120 that includes electronic circuitry. It will be understood by those having skill in the art that some AC loads, such as AC motors, can directly operate on the AC current that is provided by the UPS. However, when the AC load includes electronic circuitry 140, the electronic circuitry typically operates from one or more direct current (DC) operational voltages. Thus, as shown in FIG. 1B, the electronic circuitry 140 operates from three DC operational voltages 138a-138c at -5 volts DC, +5 volts DC, and -12 volts DC respectively. Other DC operational voltages may be used. It will be understood that electronic circuitry 140 can be a personal computer, workstation, minicomputer, mainframe computer, or any other consumer, commercial, or military electronic product.
In order to supply the DC operational voltages 138a-138c, a power supply 130 converts the AC current produced by UPS 110 into the DC operational voltages 138a-138c for electronic circuitry 140. Accordingly, power supply 130 typically includes input rectification, for example as provided by full-wave rectifying diodes 132, that produce a rectified voltage. It will also be understood by those having skill in the art that half-wave input rectification or other forms of input for rectification may also be used. A capacitor 134 may be used to filter the rectified voltage from full-wave rectifying diodes 132. The filtered voltage is provided to DC to DC converter 136. DC to DC converter 136 may include a boost converter and/or a high frequency converter, as is well known to those having skill in the art. A boost converter may be included to provide power factor correction and to reduce total harmonic distortion in the input AC line current. The design of power supply 130 is well known to those having skill in the art and need not be described further herein.
UPSs may be generally classified into online UPSs and standby UPSs. In an online UPS, a battery is used to power the AC load via a DC to AC inverter. An AC to DC converter (also referred to as a "charger") maintains the battery in its charged state. Since the battery is always powering the AC load, there need be no transition when the AC utility input voltage fails. Moreover, the battery can filter distortion or noise in the AC utility input voltage to thereby reduce "let through" to the AC load. Unfortunately, online UPSs may require a large battery, an AC to DC converter and a DC to AC inverter. Accordingly, online UPSs may be expensive and bulky.
In contrast, a standby UPS powers the AC load from the AC utility input voltage until the AC utility input voltage fails. A battery and inverter are then switched in to thereby power the AC load. The battery and inverter therefore only power the AC load on a standby basis.
FIG. 2 is a block diagram of a conventional standby UPS. As shown in FIG. 2, standby UPS 110' accepts the AC utility input voltage 100 from first plug 102 and feeds the AC utility input voltage to an AC load via the second plug 112 through a first switch 202. The first switch 202 may be a relay and/or a thyristor (triac) that remains closed as long as the AC utility input voltage is supplied to the first plug 102. Upon loss of the AC utility input voltage 100, the first switch 202 is opened. The first switch 202 may open due to its normally open configuration, or under the control of controller 214. The second switch 204 is then closed by controller 214. By closing the second switch 204, a DC to AC inverter 212 provides AC power from the battery 210 to the AC load 120 via the second plug 112. When the AC utility input voltage is restored to the first plug 102, the second switch 204 opens and the first switch 202 closes, thereby disconnecting the battery 210. The opening and closing of the first switch 202 and the second switch 204 may be controlled by controller 214 upon sensing the loss and restoration of the AC utility input voltage 100 via a sensing line 216. Other arrangements may also be used as is well known to those having skill in the art. A charger 206 maintains the battery 210 in the charged state. The design and operation of a conventional standby UPS 110' is well known to those having skill in the art, and need not be described further herein.
Unfortunately, a conventional standby UPS 110' may have many shortcomings. For example, in order to avoid backfeed from the standby UPS 110' into the AC utility input voltage 100 via the first plug 102, a delay is preferably applied by the controller 214 so that the first switch 202 opens before the second switch 204 is closed. This makes the switching from the AC utility input voltage 100 to the standby battery 210 a "break before make" transition. The AC load 120 will generally be unpowered during this transition. Moreover, switch 204 may need to be a high-voltage bidirectional switch that is able to block peak input line surge voltage in addition to blocking the inverter voltage. Finally, the DC to AC inverter 212 that supplies power from the battery 210 may need to be fully rated on a continuous basis and may need to be capable of supplying the surge power demands of the AC load on a short-term basis. The DC to AC inverter 212 may also need to provide isolation between the battery and load when the battery is low voltage and ground referenced. Accordingly, the DC to AC inverter 212 may be costly and unreliable.
Examples of online UPSs that include a high-frequency resonant converter are illustrated in U.S. Pat. No. 5,291,383 to the present inventor Oughton that is assigned to the assignee of the present invention. In this patent, FIG. 1 illustrates a block diagram of a UPS system. The UPS system comprises a rectifier connectable to an AC utility power source. The rectifier provides a DC voltage to an input filter, which in turn provides unregulated DC voltage to the input of a high-frequency resonant converter. The resonant converter provides regulated AC voltage at its output to an isolation power transformer. The power transformer includes a primary winding and a secondary winding, with the primary winding and the secondary winding coupling the converter to the rectifier which, through an output filter, supplies DC voltage to a pulse width modulation (PWM) inverter. The PWM inverter furnishes an AC voltage to the load connected to the UPS system through a low pass filter. Appropriate control circuitry is provided for control of the PWM inverter and for control of the resonant converter. The UPS system also includes a battery connectible to the input of the converter through a switch, and a charger for the battery, which charger is also connected to an AC source. See the Oughton patent, Column 1, lines 13-36. See also U.S. Pat. No. 5,057,698 to Widener and the present inventor Oughton.
The above description indicates that UPSs can be costly, complicated and bulky, and prone to reliability problems. Standby UPSs also may not provide seamless transition to the AC load upon loss of the AC utility input voltage.